1. Field of the Invention
The invention relates to a repeater for radio signals, preferably for mobile applications in digital, cellular radio networks.
A repeater is a type of relay station which receives, amplifies and retransmits the radio signals emitted by the base station of a radio network so that they can be received by the mobile stations in the radio network. Of course, a repeater also works in the opposite direction, i.e. the radio signals emitted by a mobile station are forwarded by the repeater to a base station in the radio network. In cellular radio networks repeaters are often used to expand the range of service, for example to service tunnels, large buildings, mountain valleys, etc. The use of repeaters is particularly advantageous when a line connection to a conventional base station is not possible or possible only with excessive effort because of the lack of infrastructure. Repeaters also exist for mobile use, particularly for the use in trains.
2. Description of the Related Art
The principle of a conventional repeater is the bi-directional amplification of radio signals in uplink and downlink direction, where the radio signals are received and sent on the same frequency. The downlink signal originating at the base station is received with a connection antenna, amplified and filtered in the downlink branch of the repeater and transmitted via a service antenna in the direction of the mobile station. At the same time, the uplink signal originating at the mobile station is received by the service antenna, amplified and filtered in the uplink branch of the repeater and transmitted to the base station via the connection antenna.
When used in vehicles, such as express trains, broad band repeaters transmitting a wide range of the frequencies used in the radio network are required so as to ensure proper function in each cell that is passed. The repeater""s broad band working method naturally causes signal distortions (phase and amplitude errors, intermodulation, noise, etc.) which have a very negative effect on the quality of the radio connection.
The present invention provides a mobile repeater for relaying radio signals between mobile stations and base stations of cellular radio network. The mobile repeater includes a tunable downlink branch, a tunable uplink branch, and an intelligent control unit. The tunable downlink branch receives and demodulates data streams from base station radio signals and modulates, amplifies, and retransmits the data streams as repeater radio signals in network standard format to the mobile station. The tunable uplink branch receives and demodulates data streams from mobile station radio signals and modulates, amplifies, and retransmits the data streams as repeater radio signals in network standard format to the base station. The intelligent control unit is coupled to the uplink and downlink branches and monitors signaling traffic components of the data streams. The intelligent control unit tunes the uplink and downlink branches from frequency channels used for a first base station to frequency channels used for a second base station to support handover of the relay connection from the first to the second base station.
It is the object of the invention to further develop a repeater of the above described type so as to reduce the loss of quality in the processed radio signals to a minimum and to ensure a trouble-free, mobile application of the repeater.
The problem is solved with the characteristics of patent claim 1.
The object of the invention is a repeater which demodulates and subsequently remodulates the received signals and selects the frequencies to be repeated.
The advantage of the invention is that the unavoidable noise in analog repeaters is prevented, thus significantly improving the quality of the radio connection. A further significant advantage is that the intelligent control unit of the invention recognizes the need for a change in channels as a result of the cell transition which significantly simplifies the handover process and makes the repeater particularly suitable for use in vehicles.
The repeater of the invention works according to the following functional principle:
As in a radio station of the respective radio network the signal received is filtered, amplified and demodulated (mobile station or base station). Preferably, the receiving field intensity is measured and used as the control signal for controlling the output of the transmission amplifier. In radio networks operating with TDMA (Time Division Multiple Access) the receiving field intensity is measured on a time slot basis. The demodulated, digital data stream is supplied to a modulator, amplified and then retransmitted.
In addition, in TDMA systems the burst sides (power ramping) are formed system-specific in order to obtain the smallest possible switching spectrum. The burst amplitude is controlled by the measured receiving field intensity. To stabilize the amplitude control with respect to fading interference the receiving signal may be averaged over several time frames.
Because the repeater is able to function only in selected channels the use in vehicles requires an adaptation to the respective cell situation, i.e. the frequency channels used in the cell. This is achieved in that the downlink signaling is monitored, i.e. the signals from the base station to the mobile station.
Because of the intelligent control unit in the repeater the latter needs to treat (receive, demodulate and modulate) only the frequencies of that base station (radio cell) in the respective radio network whose signals must be the strongest, in addition to the frequency of the organization channel of the next strongest adjacent cell (which is determined by the repeater""s logic).
For this purpose, the intelligent control unit of the repeater is required to monitor the signaling traffic and to extract the following information therefrom:
1. the list of the frequencies used in the strongest cell (serving cell),
2. the list of the organization channels in the adjacent cell,
3. also, depending on the radio system, information on the sequence of frequency hopping including its actual process.
When the repeater moves through a cell its control unit must be capable of making an independent decision about the imminent transition into a new radio cell and select the most suitable cell. If a cell transition (handover) is required, the repeater decreases the level of the strongest cell (serving cell) on the service side and increases the level of the target cell for the transition so that the control system of the radio system automatically initiates the transition of the mobile station connections to the new cell. As soon as the first mobile station served via the repeater has been switched to the new cell, the repeater must be capable of also serving the frequencies of the new, stronger cell. The information on whether a mobile station of the repeated cell is serviced via the repeater or via direct radio contact to the base station is determined by means of the time relation between both directions of the radio traffic and by means of the output of the signal from the mobile station which is received by the repeater.
For each repeater branch the functional units, such as channel filter, demodulator, modulator and transmission amplifier are multiple switched in parallel in accordance with the number of high frequency channels, if applicable.
The repeater receives a frequency standard which is advantageously synchronized via the synchronization channel of the downlink channel coming from the base station. This frequency standard serves as a central clock generator for generating the carrier frequency, the modulation and the burst forming, if applicable, etc.
Remote control and remote monitoring is achieved via a data connection in the form of a radio channel between the repeater and the base station. The radio channel is a component of the channels used by the repeater. The data connection is achieved by means of a componentry which has the functionality of a mobile station with data ability and which is part of the intelligent control unit. The latter may be coupled either directly to the connection antenna or it may be coupled, via a multiplexer/demultiplexer, to the digital data streams of the two repeater branches and access these.